John is writing to a lady and her children to remind them to love each other, because there are many antichrists in this world who don't believe Jesus was flesh and blood. No, really, this was a source of huge debate according to Bart D. Erhman: was Jesus really flesh, or spirit made to look like flesh? This is what the top minds in Europe thought about for hundreds of years. Imagine where we'd be if they'd put all that energy into cancer research or combating poverty. Anyway, John is clearly in the 'real flesh' section and promises that anyone on team 'fake flesh' is going to hell, so he urges his followers not to let them into their houses. He promises to come and visit soon.
John now turns his attention to someone called Gaius, about whom he heard some good things that inspired him to write. He praises him for his evanglising and for not accepting gifts from non-christians. He also passes along some gossip about a church run by one of his political enemies, a man named Diotrephes, who is trying to take over church leadership and who he unironically accuses of talking smack about him. He urges Gaius not to be swayed by Diotrephes so he can go to heaven.
And that's it. Two shortest books in the New Testament.